In general, a hot rolled steel strip is manufactured by heating a slab to a predetermined temperature in a reheating furnace, hot rolling the heated slab into a sheet bar having a predetermined thickness using a roughing mill, hot rolling the sheet bar into a steel strip having a predetermined thickness using a finishing mill having a plurality of rolling stands, transferring and cooling the hot rolled steel strip on a run-out table using a cooling apparatus, and then coiling the steel strip on a coiler. The run-out table is a transfer apparatus provided downstream of the finishing mill to transfer the hot rolled steel strip on a plurality of transfer rollers disposed at a suitable pitch.
A conventional cooling apparatus provided on the run-out table is so contrived as to mainly aim stable transfer of steel strip, as typically shown in FIGS. 1A and 1B. FIG. 1A is a schematic view of such a cooling apparatus and FIG. 1B is a lateral view of the apparatus shown in FIG. 1A. As shown in FIG. 1A, the top surface cooling of a steel strip 9 is carried out by sprinkling laminar flow cooling water 32 from laminar flow cooling nozzles 31 in cylindrical pipes which are linearly provided directly above transfer rollers 7 in the width direction of the steel strip 9 in such a way that the steel strip 9 does not undulate on the transfer line due to water pressure. On the other hand, as shown in FIG. 1B, the bottom surface cooling of the steel strip 9 is carried out by intermittently jetting cooling water 34 from spray nozzles 33 provided between the transfer rollers 7 to the steel strip 9.
Recently, excellent workability, high strength with low carbon equivalent and the like have been required for a hot rolled steel strip. For these requirements, grain refining of steel strip is effective, and thus the steel strip need to be more rapidly cooled after hot rolling. In particular, the steel strip having low carbon equivalent such as an ultra low carbon steel strip should be cooled at a cooling rate exceeding 200° C./s because austenitic grains after hot rolling tend to become coarse due to recrystallization.
To conduct such rapid cooling, Japanese Unexamined Patent Application Publication No. 62-259610 discloses a method for increasing cooling capability for bottom surface of steel strip using a bottom surface cooling apparatus where cooling water jetting plates having a plurality of holes are disposed between transfer rollers and also function as a guide, and jetting cooling water toward the steel strip through the holes at different angles.
However, the method described in Japanese Unexamined Patent Application Publication No. 62-259610 causes various problems as follows.
(1) A hot rolled steel strip undulates vertically while being transferred on a run-out table when the leading end of the hot rolled steel strip lies between a finishing mill and a coiler, because the hot rolled steel strip is not under any tension. Cooling of such a tension free steel strip in this method causes further vertical waves. As a result, a sufficient volume of cooling water is not applied and it is impossible to cool, for example, a steel strip of 3 mm in thickness at a cooling rate exceeding 200° C./s.
(2) This method does not enable the top and bottom surfaces of the steel strip to be cooled at the same cooling rate.
(3) This method presupposes cooling at a water flow rate of about 1,000 L/min·m2, but a higher water flow rate is required to cool a steel strip of, for example, about 3 mm in thickness at a cooling rate exceeding 200° C./s. In the cooling apparatus used in this method, as shown schematically in FIG. 2A, a higher water flow rate causes jetted cooling water to remain in a narrow space between the cooling water jetting plate and the steel strip around the center in the width direction of the steel strip. Therefore, desired cooling is not performed because of a decrease in the flow velocity of the jetted cooling water. On the contrary, around the edge in the width direction of the steel strip, the cooling water flows down from the edge without remaining and therefore allows desired cooling. As a result, as shown in FIG. 2B, the temperature profile in the width direction of the steel strip shows an inverted-V shape, in which both edges are cooled to target temperature but the center is cooled to temperature higher than the target temperature. Thus, uniform cooling in the width direction is not performed.
Widening the space between the cooling water jetting plate and the steel strip, as shown in FIG. 3A, prevents cooling water from remaining at the center in the width direction of the steel strip, performing desired cooling. However, a large amount of cooling water is drained from the center toward the edges in the width direction of the steel strip after cooling, disrupting the cooling water flow at the edge in the width direction to lower cooling capability. As a result, as shown in FIG. 3B, the temperature profile in the width direction of the steel strip shows a V shape, in which both edges are cooled to temperature higher than target temperature and the center is cooled to the target temperature. Thus, uniform cooling in the width direction is not performed.
When the space between the cooling water jetting plate functioning also as a guide and the steel strip is arranged properly, the temperature profile in the width direction of the steel strip after cooling shows an M shape which is the sum of the inverted-V shape in FIG. 2B and the V shape in FIG. 3B. Thus, uniform cooling in the width direction is not performed, either.
(4) According to this method, when the cooling water is jetted toward the steel strip at different angles from a plurality of holes in the cooling water jetting plate functioning as nozzles, the distance that the cooling water travels varies depending on the nozzles. The cooling water jetted aslant to the steel strip travels a longer distance, thus greatly reducing the flow velocity to fail to efficiently cool the steel strip. As described in (3), cooling capability is greatly affected by the jetted cooling water, so it is more difficult to uniformly cool the steel strip in the width direction.